The Health, Aging, and Body Composition Study (Health ABC) is a longitudinal cohort of 3,075 men and women, aged 70-79 at baseline in 1997-98, and includes 552 African American men and 729 African American women. At baseline, all cohort members reported no difficulty walking 1/4 mile or up ten steps. The study sites are Memphis, Tennessee and Pittsburgh, Pennsylvania. The study was designed to investigate how changes in body composition (muscle, fat, and bone) act as a common pathway through which weight-related health conditions and behavioral factors contribute to loss of function. Race and gender differences and similarities over the 16 years of the study have been key to understanding health trajectories as the cohort has aged. The study measurements include weight-related health conditions such as diabetes and metabolic disorders, osteoporosis, osteoarthritis of the knee, cardiovascular disease, pulmonary disease, and depression. Body composition was repeatedly measured with both dual energy x-ray absorptiometry and computerized tomography. We also assessed muscle strength, and physical function, as well as an extensive set of biomarkers. The major outcome of the study is persistent difficulty walking 1/4 mile or stair climbing reported over a 6-month period. Other outcomes include cardiovascular events, cancers, fractures, dementia, diabetes, hospitalizations, and deaths. The study has 17 years of follow-up through clinical exams and telephone follow-up interviews. A capstone clinic examination started in year 16 for surviving cohort members. The year 16 exams in Pittsburgh were conducted in a subset who were eligible for a muscle-tissue biopsy. Memphis capstone exams included the standard measures from earlier clinic exams. Home visits at year 16 were also done in Memphis. All year 16 exams have added activity monitoring via accelerometers on the wrist and hip to objectively track the everyday activity level (or inactivity level) of participants over 7 days in a free-living environment. Numerous grant-funded ancillary studies have been added by extramural investigators to enhance the interdisciplinary contribution of Health ABC to the research community.